1. Field of the Invention
This invention relates to a capacitive device such as a variable capacitor controlled using the <<Coulomb blocking>> phenomenon.
The Coulomb blocking phenomenon occurs in conducting or semi-conducting aggregates on which electrical charges can be accumulated, when isolated from their environment and slightly coupled to it due to the tunnel effect. In particular, this phenomenon characterizes some metal-insulator-metal junctions composed of two metallic armatures L, R separated by a dielectric matrix D in which small metallic particles or aggregates, typically with a size of a few nanometers, are dispersed. FIG. 1 diagraminatically shows a cross-section through such a structure. Coulomb blocking can be seen firstly by zero conductance between the two armatures L, R, or electrodes, below a voltage threshold Vs. It simply represents the electrostatic energy to be overcome to make an electron transit through a central aggregate and consequently to circulate a current from one armature to the other.
The curve in FIG. 2 shows the charge N accepted by an aggregate for a given voltage, as a function of the threshold voltage Vs applied at the armatures of the capacitor. For a given energy threshold, the charge of the aggregate is incremented by one unit from Ne to (N+1)e, which can be seen on the I(V) characteristic of the tunnel junction by a series of constant parts and steps, the steps corresponding to voltages at uniform intervals in units of Vs.
These blocking effects are very well described at low temperature, but have to be weighted by the thermal activation energy (kT=25 meV at 300 K) for devices operating at ambient temperature.
2. Description of the Related Art
Prior art divulges capacitors with variable semi-conductor type control, that have the disadvantage of relatively large leakage resistances. These leakage resistances may be harmful to smooth operation of the capacitor, and for example the quality factor may be degraded.
The purpose of the invention is a capacitive device such as a voltage controlled variable capacitor, for example using a DC voltage.
The concept is to be able to control the quantity of charges stored on aggregates and on each armature in the capacitor, by means of a potential difference. The capacitor may be considered as the ratio between the quantity of charges stored on the armatures and the voltage that can be modulated as a result.